成人快手

Knots fly by rowing themselves through the air, stretching their wings forward and beating them downwards. On the upstroke the bird half-folds them to reduce their surface area and therefore their resistance to the air. The feathers on its wings slide smoothly over one another to keep its surface smooth and streamlined. Its body is also streamlined by its coat of feathers, and it keeps its feet tucked against its tail to minimize drag. A mallard can fly at 40 miles per hour but its flying is so streamlined that it hardly ruffles a feather. It is only from behind that one notices the flicks of its tail and the back of its wings. An osprey demonstrates the importance of streamlining after it dives into the water to catch a fish - it has to use all its strength to get back in the air. With a fishing hanging broadside from its claws, an osprey makes hardly any headway at all. So the bird alters its grip to point the fish forwards and takes advantage of the fish's own streamlining. Once it has done this, the effort of flying is much reduced.